Radio frequency (RF) transceivers are found in many one-way and two-way communication devices, such as portable communication devices, (cellular telephones), personal digital assistants (PDAs) and other communication devices. A modern RF transceiver must be capable of operating using a number of different frequency plans and communication methodologies, which are dictated by the particular communication system within which the transceiver is operating.
However, supporting multiple communication bands on a single RF chip increases the complexity of such systems. As the complexity grows, a major concern is minimizing the power consumption of the radio. Therefore, recent implementations focus mainly on reduction in complexity, cost and power. As an example, in a complementary metal oxide semiconductor (CMOS) based transceiver for quad band cellular applications, the phase noise requirement for the Global System for Mobile Communications (GSM) is −162 dBc/Hz at 20 MHz away from the carrier for the transmitted signal. This requirement translates to increased power consumption in the local oscillator (LO) generation chain.
The center frequency of the transmitted mask in the GSM850 communication band varies from 824-849 MHz and it varies from 890-915 MHz in the GSM900 communication band. However, the voltage controlled oscillator (VCO), which generates the transmit center frequency, is designed to oscillate at a multiple of the transmitted frequency (usually 2 or 4 times, depending on the transmit band) in order to reduce the effect of LO direct feed-through. Then a series of dividers converts the generated frequency to the desired frequency. One example of a frequency plan uses two consecutive divide-by-two elements after a VCO operating at approximately 3-4 GHz to obtain the desired transmit frequency.
It is desirable to minimize the phase noise contributed by the dividers and buffers used in the frequency divider to the final phase noise of the generated low band and high band frequency tones. For applications such as a CMOS based GSM transceiver, where the transmitted signal phase noise requirement is stringent, this requirement mandates high current consumption in the sequential dividers that are used to develop the transmit frequency. This in turn will undesirably raise the overall power consumption of the radio transceiver.
Therefore, it is desirable to minimize the amount of phase noise contributed by a frequency divider.